Electrical and electronic conductivity of CaO-SiO 2 -FeO x slags at various oxygen potentials: Part II. Mechanism and a
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ATURE REVIEW
THERE are a considerable number of experimental studies and empirical correlations on the electrical properties of iron oxide–containing melts in equilibrium with iron.[1–4] In contrast, the amount of published work on the electrical conductivity of iron oxide–containing melts at more oxidized conditions is very limited. These include articles by Engell and Vygen,[5] Pastukhov et al.,[6] Fontana et al.,[7] and more recently by Hundermark et al.[8,9] Most of the work on this topic has been aimed at obtaining conductivity data for the optimization of nonferrous smelting/melting furnaces. In these studies, the explanation of experimental measurements has been made by dividing total conductivity into an ionic (i) and an electronic (e) conductance, as shown in Eq. [1]. s si se
[1]
the exponent power, m, was approximately constant, taking values between 0.11 and 0.14. A theoretical value of m 1/6 was calculated by Pastukhov assuming that the concentration of electron holes () as charge carriers is established via the following reaction: 1 O2 FeO Fe2 2 2
where Fe2 represents an iron ion deficit site. For FeOx-CaO-SiO2 melts with CaO/SiO2 0.79 at 1873 K and variable oxygen pressures, Engell and Vygen[5] divided the electrical conductivity to ionic and electronic parts, as shown in Eq. [1]. In contrast to Pastukhov et al., they suggested that the ionic conductance is also dependent on the oxidation state of the iron and can be expressed as si s0 ax
[6]
Pastukhov et al. found that in FeOx-SiO2 melts, electronic conductivity becomes dominant at FeOx higher than approximately 70 wt pct, while for FeOx-(40CaO-40SiO220Al2O3 wt pct) slags, this transition takes place at FeOx greater than approximately 20 wt pct. In slags exhibiting significant electronic conductivity, total conductance reached a maximum at a certain oxygen partial pressure, which was defined by the slag composition and temperature. In their analysis, Pastukhov et al. regarded the ionic conductance as being independent of oxygen potential. The electronic conductance, on the other hand, was correlated to the partial pressure of oxygen as se kpOm2
[2]
In this empirical expression, the coefficient k was found to be dependent on slag composition and temperature, whereas MANSOOR BARATI, former Ph.D. student, McMaster University, is currently with WP HG Engineering, Toronto, Ontario, Canada. KENNETH S. COLEY, Associate Professor, is with Department of Materials Science and Engineering, McMaster University, Hamilton, Ontario, Canada L8S 4L7. Contact e-mail: [email protected] Manuscript submitted June 11, 2005. METALLURGICAL AND MATERIALS TRANSACTIONS B
[3]
[4]
where 0 is a constant for a particular slag and x is the fraction of the iron ions in the higher oxidation state, i.e., ferric ion. The electronic part of the conductivity was related to x through the following equation: se bx(1 x)
[5]
where the coefficients a and b are constant for a particular slag and could be determined from experiments. Fontana et al.[7]
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